The Living Sea of Waking Dreams by Richard Flanagan

Anna's aged mother is dying - if her three children would just allow it. Forced by their pity to stay alive, she increasingly escapes through her hospital window into visions of horror and delight. When Anna's finger vanishes and a few months later her knee disappears, Anna too feels the pull of the window. She begins to see that all around her others are similarly vanishing, but no one else notices. All Anna can do is keep her mother alive.